In open-loop amplifier architectures, there is no external feedback to the amplifier control signal during operation of the amplifier. Open-loop amplifier output control is desirable in some applications because it has reduced cost and reduced complexity compared to closed-loop amplifier control architectures. For example, open-loop amplifiers permit the elimination of the coupler and detector used commonly at the amplifier output in closed-loop feedback systems. The coupler and detector add cost and consume power.
One contributor to battery power consumption in battery-operated wireless cellular telephones is the current (Id) drawn by the power amplifier during transmission. The current (Id) drain is generally a function of the output impedance presented to the power amplifier. Any detuning of the antenna however causes both the magnitude and phase of the load impedance presented to the power amplifier to change, which may be characterized in terms of impedance mismatch relative to a reference impedance. In cellular telephones, for example, antenna detuning occurs as the users handles the telephone, contacts the antenna, etc. The load impedance is also a function of the frequency of the signals transmitted. Thus for a particular output power (Pout) the current (Id) drain varies depending on the phase of the impedance mismatch.
In open-loop power control transmitter architectures using collector voltage control, the base bias of the amplifier is fixed while the collector voltage is set as a function of the output power. When the output impedance changes, the output power and the current drain may vary exceedingly resulting possibly in out-of-specification performance and excess power consumption.
The various aspects, features and advantages of the disclosure will become more fully apparent to those having ordinary skill in the art upon careful consideration of the following Detailed Description thereof with the accompanying drawings described below.